Condensation is needed for high efficiency boilers and it only occurs if the flue gas temperature is below 120deg F, so to lower the temperature they bring in cold winter air (which obviously defeats any heat savings) when the boiler is running extra hot in winter.
READ BELOW. (*****the lower the stack temperature the more effictive the heat exchange.....****)
QUOTE:
"Stack Losses:
Stack temperature is a measure of the heat carried away by dry flue gases and the moisture loss. It is a good indicator of boiler efficiency. The stack temperature is the temperature of the combustion gases (dry and water vapor) leaving the boiler and reflects the energy that did not transfer from the fuel to the steam or hot water. The lower the stack temperature, the more effective the heat exchanger design, and the higher the fuel-to-steam efficiency."